Design Guidelines for Energy Efficient Power Supplies

In recent years, with the intensification of energy shortage and global warming, environmental awareness has become a consensus among consumers, who are increasingly concerned about small size, multi-function, energy saving and power saving. For high-efficiency power supplies, it is necessary to make full use of electrical energy and reduce unnecessary energy consumption or loss as much as possible. This kind of power supply design that meets environmental protection requirements is what consumers are happy to accept.

Energy efficiency specifications guide the design direction of high-efficiency power supplies

Today, governments of various countries are also vigorously advocating the sustainable development of energy and have put forward various environmental protection directives. Driven by energy efficiency regulations and environmental awareness, the power supply market is undergoing tremendous changes. According to the latest survey, the largest demand comes from computer power supplies, followed by LCD TVs, electronic ballasts, and adapter power supplies. With the continuous development of the computer, LCD TV, and laptop markets, the demand for high-efficiency power supplies for these products is also increasing day by day.

In addition, government agencies and industry organizations around the world have formulated corresponding energy consumption specifications and standards. In addition to "80 PLUS", the industry has also put forward newer energy-saving requirements for computer power supplies. In recent years, the Climate Change Initiative of the Computing Industry (CSCI) has put forward higher energy-saving requirements, that is, computer power supplies must meet 80% or higher energy efficiency standards and requirements under 20%, 50% and 100% load conditions. In addition, the Energy Star TV product version 3.0 specification and the Energy Star external power supply version 2.0 specification launched in the United States in 2008 and the Energy Star (ENERGY STAR?) computer version 5.0 specification launched in 2009 are also aimed at improving the utilization rate of electrical energy by equipment. The increasing popularity of energy efficiency standards requires that power conversion in all operating modes have higher energy efficiency, including reducing standby (no-load) energy consumption, improving power supply efficiency, using power factor correction (PFC) or reducing harmonics.

Examples of methods to improve circuit segment energy efficiency in power supply design

In a power supply design, the challenges of energy efficiency regulations to circuit segments are mainly reflected in PFC energy efficiency, main converter energy efficiency and secondary energy efficiency. To improve the energy efficiency in these aspects, it is necessary to improve the performance of the components in the red box of the circuit shown in Figure 1. Several methods for improving the energy efficiency of circuit segments are described below.

1. Improve PFC energy efficiency

The goal of improving PFC energy efficiency is to achieve a power factor correction efficiency of more than 96%. Bridgeless PFC (Figure 2a) can reduce bridge losses, and interleaved PFC (Figure 2b) can meet the requirements of higher power applications to improve PFC energy efficiency. In addition, IC technology can be used to reduce switching losses, and more optimized topology structures can be used to reduce EMI filter losses.

Using ON Semiconductor's NCP1605 high-efficiency standby mode PFC controller can improve PFC light-load efficiency and further reduce losses. The device uses a high-voltage current source, an externally set fixed switching frequency, and can operate in DCM/CRM mode; it can work in a soft-skip cycle (Soft-SkipTM) under standby conditions; the PFC ready signal can perform fast line/load transient compensation; valley conduction can achieve overvoltage protection and undervoltage protection; it also has input undervoltage detection, soft start for smooth start, overcurrent limit and latch functions.

2. Improve the efficiency of the main converter section

There are several ways to improve the efficiency of the main converter. First, reduce the primary conduction loss by reducing the on-resistance (higher switching losses) and/or reducing the primary peak current and RMS current; second, consider using soft switching technology to reduce switching losses; third, reduce secondary losses by reducing the rectifier voltage drop (using low forward voltage drop diodes or FET rectifiers); fourth, use better core materials to reduce core losses.

Table 1 lists the main soft switch topologies for reference in design.

Improve secondary energy efficiency

Synchronous buck converters are a good solution to improve energy efficiency, and DC-DC soft switching technology can further improve energy efficiency. The full-load energy efficiency of ON Semiconductor's NCP4302 synchronous buck controller is 2.5% higher than that of Schottky diodes; and the NCP4331 post regulator is 7% higher than that of traditional magnetic amplifiers, both of which can contribute to the improvement of secondary energy efficiency.

Preview of application examples of high-efficiency power supply products:

In order to explain the essence of high-efficiency power supply design in more detail and systematically, we will provide you with several typical high-efficiency power supply reference designs in the next few lectures, including meeting the trend of personal computer power supply energy efficiency and improving the energy efficiency of LCD TVs. We will also tell you about solar charging controller design and high-efficiency smart meter power supply solutions.

1. Laptop power adapter design

With more and more notebook users demanding high-performance, small size or low weight notebooks at a reasonable price. For power adapter designers, it is necessary to choose a suitable controller to develop high-efficiency, integrated with rich protection features, and compact adapters. Judging from the usage of most users, laptops are in light load or standby conditions for a considerable period of time. Compared with improving energy efficiency under 25%, 50%, 75% or 100% load conditions, reducing energy consumption and improving energy efficiency under extremely low load conditions or even standby conditions is more challenging. This requires the power controller to have excellent light load or standby energy consumption performance.

In the second lecture, we will explain the design example of using NCP1250/NCP1251 flyback controller to produce a power adapter that meets the requirements of laptop computers. Because NCP1250/1 integrates key features required for high-density power adapter applications, such as non-dissipative over-power protection, over-temperature protection, small package (TSOP6) and Vcc pin overvoltage protection (OVP) (only NCP1251). While providing high working efficiency, NCP1250/1 uses frequency reflux technology and skip cycle mode to achieve extremely low energy consumption in light load or standby mode, thereby providing high energy efficiency over the full load range.

2. High-efficiency power controller solutions for desktop computers and flat-panel TVs

Computers, servers and flat-panel TVs have always been important targets for energy efficiency regulatory agencies. These devices must meet the latest energy efficiency requirements while meeting high performance. In the third lecture, we will introduce the main features and application design points of the power supply segment of the high-efficiency, high-performance power factor correction (PFC) and half-bridge resonant dual-inductor plus single capacitor (LLC) combination controller NCP1910 used for computer ATX power supplies and flat-panel TVs, to help engineers better use NCP1910 for related power supply designs.

3. High-efficiency solar charge controller design

The fourth lecture will introduce a solar cell controller NCP1294, which is used to implement maximum peak power point tracking (MPPT) of solar panels to charge batteries with the highest energy efficiency. Since NCP1294 is a fixed-frequency voltage mode PWM feedforward controller, it contains all the basic functions required for voltage mode operation. As a charging controller that supports different topologies such as buck, boost, buck-boost and flyback, the NCP1294 is optimized for high-frequency primary-side control operations, has pulse-by-pulse current limiting and bidirectional synchronization functions, and supports solar panels with a maximum power of 140 W. The MPPT function provided by this device can locate the maximum power point and adjust it in real time according to environmental conditions, so that the controller remains close to the maximum power point, thereby extracting the maximum amount of electricity from the solar panel and providing the best energy efficiency.

4. High-efficiency smart meter power supply solution

From the perspective of the composition of smart meters, it mainly includes functional modules such as communication, power supply and power management, metering and storage. As for the power supply and power management module, the content of the fourth lecture will provide you with design solutions including high-voltage AC-DC switching regulators, DC-DC switching regulators/controllers and low-dropout (LDO) linear regulators, so that readers can choose the appropriate solution according to specific applications. These power supply solutions have the characteristics of high energy efficiency, low energy consumption and rich protection features, and will become one of the future development trends of smart meters.